The present invention relates to techniques for transmitting digitized images, and in particular to a technique for providing more rapid transmission of critical image information.
In the 1960s, the Department of Defense developed a distributed network of telephone lines linking computer systems known as the Internet. The Internet was first used primarily for transmitting text information, but in 1991, British scientist, Tim Berners-Lee invented the World Wide Web, which allowed images to be readily transmitted over the Internet along with or instead of text. The World Wide Web has greatly increased the amount of image information transmitted over the Internet. This increased image traffic is taxing the capacity of the Internet and the patience of its users who often must wait a considerable period of time for image transmission to be completed. The transformed image also requires less data for a complete loss-less transmission.
The transmission of images on the Internet or any similar capacity communications network is time-consuming as a result of the large amount of data represented by an image. For example, a color image displayed on a typical computer screen may have the equivalent of one hundred pages of text information. Even with present day high speed telephone lines and modems, transmission of an image may take many seconds. When a network connection is shared among many users, or if the users have slower hardware, transmission of pictures can take many minutes. In situations where multiple images need to be reviewed by a user, even modest transmission times can be unacceptable.
In order to reduce the amount of image data that must be transmitted, it is known to compress image data. Compression techniques include loss-less compression schemes in which the compressed image may be decompressed without loss of information, and lossy compression systems, where the compressed data is irreversibly degraded.
A loss-less compression system, for example, might recognize a pattern of consecutive pixels in the image and transmit a short code designating that pattern rather than transmitting the more lengthy values of the pixels themselves. A decompressing program at the receiving terminal, receiving the short code, inserts the designated pattern into the received stream of pixel data. The decompressed data can be identical to the data before compression.
A lossy compression system, as an example, may reduce the number of shades of gray of each pixel in the image, from 256 to eight. At the receiving site, the image is reconstructed in a degraded eight gray level form. Generally, lossy compression systems provide far greater compression than loss-less compression systems. In certain applications, however, lossy compression systems may be inappropriate. For example, in the transmission of diagnostic medical images, degradation of the image in a lossy compression transmission may obscure significant clinical information.
One method of addressing the problem of slow image transmission of high quality images, especially where multiple images must be reviewed by a user, is to first transmit an extremely compressed version of the image, typically a "postage stamp image" having much smaller size and less detail. A number of postage stamp images may be transmitted rapidly, viewed by the user, and one or more selected for its complete image to be transmitted in its entirety over a longer period of time. This two-step approach eliminates the need to transmit each of the images in their entirety, but entails additional overhead of transmitting the postage stamp images, data which will ultimately be discarded.
Different users of image data may have differing requirements for image quality. These different requirements may result from their different uses of the data, different limiting resolutions of the user's computer equipment, or different limiting bandwidths of the user's connections to the network. Ideally, each such user could be provided a different image having a different degree of data compression. Unfortunately, storing many different copies of the image with different degrees of compression may be impractical as a result of the large amount of memory required.